Thyristors are commonly used for high current and high voltage applications in motor drive circuits, appliance controls, robotics, lighting ballasts, solid-state relays, power inversion in high-voltage transmission lines and in other high power applications. This is because thyristors exhibit bistable characteristics and can be switched between a high-impedance, low-current OFF state and a low-impedance, high-current ON state. Thyristors are now available with current ratings from a few milliamperes to over 5000A and voltage ratings extending above 10,000 V. Triode AC switches (TRIACs) are also extensively used for high power switching applications because they are inexpensive to manufacture and offer thyristor-like characteristics in two directions and have ON and OFF states for both positive and negative anode voltages. Unfortunately, TRIACs typically require a moderate amount of gate current to trigger conduction in both first and third quadrant modes of operation. TRIACs also typically do not exhibit current saturation at high voltages or gate-controlled turn-off capability. Instead, TRIACs are typically turned off by reversing the anode potential. These features of TRIACs typically result in power devices having reduced forward-biased safe-operating-area (FBSOA) and reduced reverse-biased safe-operating-area (RBSOA). Moreover, because the rate at which TRIACs may be turned on typically cannot be carefully regulated, significant electromagnetic interference (EMI) may be encountered when such devices are used for consumer applications. The on-state voltage drop of a typical TRIAC may also be relatively large (on the order of about 1 V or higher) and such voltage drops can lead to significant power losses during high on-state forward conduction.
To address these limitations associated with conventional TRIACs, preferred bidirectional AC switching devices have been proposed. Such devices are described in U.S. Pat. No. 5,493,134 to Mehrotra et al. entitled "Bidirectional AC Switching Device With Mos-Gated Turn-On and Turn-Off Control", assigned to the present assignee, the disclosure of which is hereby incorporated herein by reference. Notwithstanding these preferred bidirectional devices, improved power semiconductor devices having carefully controllable turn-on and turn-off characteristics are still required.